Apparatus for the mass analysis of plasmas on a continuous basis



July 2, 1963 R. v. NEIDIGH 3,096,438

APPARATUS FOR THE MASS ANALYSIS OF PLASMAS ON A CONTINUOUS BASIS Filed April 24, 1961 2 Sheets-Sheet 1 HELIUM ARC NITROGEN RESIDUALJLOXYGEN I II I REF. 2816 4 MASS NO.

Fig. 3.

DEUTERIUM GAS I 4 2 MASS No.

(a) r {NITROGEN RESIDUAL OXYGEN INVENTOR. Rodger V. Neidigh BY ATTORNEY July 2, 1963 R. v. NEIDIGH 3,095,438

APPARATUS FOR THE MASS ANALYSIS OF PLASMAS ON A CONTINUOUS BASIS Filed April 24, 1961 2 Sheets-Sheet 2 OSCILLOSCOPE I i VOLTAGE INDEX I ON 2nd TRACE l /?-22 TO A-B AMPLIFIER ON DUAL BEAM Fig.

+ III I'II MAGNETIC FIELD OUT OF PAPER INVENTOR. Rodger V. Neidigh BY ATTORNE Y United States Patent 3,096,438 APPARATUS FOR THE MASS ANALYSIS OF PLASMAS ON A CONTINUOUS BASIS Rodger V. Neidigh, Knoxville, Tenn, assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Apr. 24, 1961, Ser. No. 105,244 4 Claims. (Cl. 250-419) This invention rel-ates to an apparatus for the mass analysis of plasmas and the like on a continuous basis.

One field for usage of apparatus for the mass analysis of plasmas is in the study of the ionic composition of high current are discharges such as disclosed in US. Patents No. 2,920,234, issued January 5, 1960; No. 2,920,235, issued January 5, 1960; No. 2,927,23, issued March 1, 1960; and No. 2,928,966, issued March 15, 1960. Such high current are discharges are being utilized as a means for changing the charge-to-mass ratio of an injected molecular ion beam in a strong, confined magnetic field. The result is a magnetically trapped atomic ion plasma. By studying the ionic composition of such are discharges, it will be possible to more accurately understand the mechanism of dissociation of an injected ion beam by these arc discharges.

One method ot making such an analysis of the are discharge ionic composition or the composition of other plasma sources is by using mas-s spectrographic techniques. Ions are withdrawn from the arc discharge or plasma source with a negative potential and strike a collector plate. This plate is moved radially step-by-step to intercept sequentially the desired ionic species, with the resultant current being a representative value for the intensity of each of the desired species. This rudimentary approach is time consuming and leads to uncertainties in the data due to the length of time required to examine the full range of ionic species and the lapse of time between taking the measurements and examining the data.

With a knowledge of the limitations of the method discussed above, it is a primary object of this invention to provide an apparatus for the rapid and continuous mass analysis of the ionic composition of a plasma or the like.

It is another object of this invention to provide an apparatus for the rapid and continuous mass analysis of the ionic composition of a plasma or the like and including means for displaying continuously such ionic composition.

These and other objects and advantages of this invention will become apparent from a consideration of the following detailed description and the accompanying drawings, wherein:

FIG. 1 is a cross sectional view of a mass analyzer for accomplishing the above objects;

FIG. 2 is a schematic diagram of the electrical circuit of the mass analyzer of FIG. 1;

FIG. 3 is a reproduction of a typical oscilloscope trace showing the analyses of a helium arc discharge as obtained with the mass analyzer of FIG. 1; and,

FIG. 4 is a reproduction of typical traces showing the analysis of a deuterium arc discharge as obtained with the mass analyzer of FIG. 1.

The above objects have been accomplished in the present invention by providing a pair of parallel electrodes in a tubular member which serve as a velocity selecting region for ions drawn by an accelerating potential through a tapered nose cone affixed to the tubular member. The magnetic force and electrostatic forces in the velocity selecting region are made equal and opposite in direction to prevent the ionic species from striking either of the electrodes as they traverse the region. A pair of parallel plates are positioned within the tubular member and in 3,096,438 Patented July 2, 1963 ICC alignment with the electrodes, but displaced slightly so as not to be seen by dire-ct light coming through the entrance slit, and one of these plates serves as a collector plate. This collector plate is coupled to the vertical amplifier of an oscilloscope or other recorder.

FIG. 1 illustrates one embodiment in which the principles of this invention may be carried out. An exterior shell 1 is a cylindrical copper tube with an outside diameter of about 2 /8 inches. The shape of the device permits its insertion through conventional vacuum seals, not shown, of devices which produce ions of which an ionic mass analysis is desirable. The operational end of the shell is substantially closed with a flattened tungsten nose cone 2 containing a inch wide slot 3 in the apex thereof. An inner cylindrical copper sleeve 4 is positioned coaxially within shell 1 with an annular insulator 5 at one end thereof and a sleeve, not shown, at the other end.

Positioned within the inner sleeve 4 is a pair of confronting parallel plate tantalum electrodes 6, 7. Electrode 6 is supported on an insulator 8, and electrode 7 is mounted on a conducting support 9. These electrodes project out from the inner sleeve 4 toward the slit 3 and are positioned from about /8 to inch from the slit. The gap between the electrodes 6, 7 is aligned along the axis of the spectrometer and with the slit 3. Also positioned within the inner sleeve 4- is .a second pair or" confronting parallel plate copper electrodes 10, 11 which, in turn, are parallel to electrodes 6, 7. Electrode 10' is mounted from a conducting support 12, and electrode 11 is supported on an insulator 13, but ofiset so as to prevent light rays from the plasma from impinging on it. Cooling coils 14 are brazed to the inside of the outer shell 1 to remove a substantial portion of the heat radiated from the source of ions under analysis.

Although not shown in FIG. 1, the inner sleeve 4 is closed at its outer end remote from the nose cone 2. Electrical connections are made through vacuum-tight connectors passing through the end closure. A vacuum seal between the inner sleeve 4 and the exterior shell 1 is provided at the outer end by a gasket clamped between :a flange and a clamping ring, not shown.

The outer shell 1 of the analyzer of FIG. 1 is provided with pump out holes 15, and the inner sleeve 4 is provided with pump out holes 16. These holes 15 and 16 together with the slit 3 are utilized for evacuating the analyzer of FIG. 1, when it is inserted into the vacuum system of the device containing the arc discharge or plasma under analysis. The holes are positioned so that magnetic flux lines do not pass through them.

The electrical circuit for the mass spectrometer of FIG. 1 is shown in FIG. 2. The exterior shell 1 is grounded as is the positive terminal of a high voltage (U -5000 volts) D.C. supply 20. The negative terminal of the power supply 20 is connected to the inner sleeve 4, to the electrode 7, and to the electrode 10, as well as to one end of the secondary winding of a transformer 21. The other end of the secondary winding of transformer 21 is connected to the electrode 6. Electrode 11, which is the collector plate, is coupled to the vertical amplifier of an oscilloscope. This electrode 11 may be alternatively coupled to an oscillograph or other type of recorder, if desired. Electrode 11 is also connected through a variable load resistor 22 to the negative terminal of the power supply 20. The negative DC. voltage, as well as the variation due to the AC. voltage (supplied through the transformer 21) are also fed to the oscilloscope or other recording means.

The principle of operation of the mass spectrometer of FIG. 1 and FIG. 2 is as follows. The negative DC. potential between the nose cone 2 and the electrodes 6, 7 draws ions from the source of ions to be analyzed into the spectrometer through the slit 3. The equation of motion for the ions in the region between slit 3 and the entrance to the electrodes is:

where e is the ionic charge, In is the ionic mass, U is the accelerating potential, and v is the velocity. Since each ionic species will enter the velocity selecting region between electrodes 6, '7 with a different velocity. If the ionic species is to traverse the velocity selecting region without striking either of the electrodes 6, 7, the magnetic force and electrostatic forces thereon must be equal and oppositely directed. Thus, eE=Hev, Where E is the electric field, and H is the magnetic field. Therefore where V is the potential across the channel between electrodes 6, 7, and d is the channel spacing. The conditions of V, d, H and U, in order to accelerate an ion and permit it to pass through the channel, are established by eliminating v from the equations; and thus V2 UQPH When U, d, and H are fixed, a variation of V will analyze the incoming ions according to their respective species. This variation of V is accomplished by the A.C. voltage superimposed upon the DC. acceleration voltage. The direction of the magnetic field H is out of the paper as shown in FIG. 1. This field is normally provided by the magnetic field of the are discharge or plasma source under analysis.

With the specific conditions existing in the spectrometer as designed and used, a scan of masses covers the complete range irom mass 1 to infinity. However, masses only up to about 28 are clearly separable. In the field of thermonuclear experimental devices, however, only masses 1 through 8 are of most interest and the separation is suificient to clearly discern, for example, the mass defect between H and 13 Again, in this particular field, masses above 28 are of interest only as impurities. The mass analyzer of FIG. 1 is useful then to detect the presence of such impurities even though positive identification of these are not possible.

FIG. 3 and FIG. 4 show typical scans obtained using the mass analyzer of FIG. 1. The traces of the reference voltage are clearly seen, and the pips on the traces clearly indicate the various ionic species. The sloping lines are provided by the voltage V, applied to the velocity selector. It can be seen that the ions of primary interest in thermonuclear experimental machines are clearly distinguishable. These mass analyses were made of helium and deuterium arcs at a magnetic field strength of 5000 gauss, an accelerating voltage of 3000 volts D.C., and a channel spacing, d, of 0.2 cm. The AC. voltage was 60 cycles, 500 volts peak-to-peak. The nose cone of the analyzer was placed about inch from the arc boundary for obtaining the above traces. This spacing of inch from the arc boundary is not critical, however, and the ionic composition of the area surrounding the arc will vary at diiferent distances from the arc boundary. Thus, it is possible with the device of FIG. 1 to analyze the ionic composition of the area surrounding the are at several selected distances from the arc boundary in the range from 1 to 1 /2 inches, for example. The heights of the pips on the oscilloscope trace will be different at each selected distance from the arc boundary because the ionic composition of the area surrounding the arc is different at each of the selected distances from the arc boundary. Of course, there will be some slight fluctuations of the pip heights at each selected distance because of the fact that an energetic arc discharge is not completely stable during its operation.

The device of PEG. 1 can be used to analyze any form of an ionized plasma. The tungsten nose cone and water cooled outer sleeve Will permit insertion into an arc plasma of considerable power density (-1 kvv./in. in a lithium arc). The range of mass at which the ions can be identified can be increased by use of appropriate potentials as specified in the above-identified equations.

The mass analyzer of this invention has the following advantages over analyzers of the prior art: (1) The overall size of the analyzer is reduced to the extent that it can be built inside 2% inch O.D. copper tubing. This permits insertion of the device through conventional vacuum locks of vacuum systems and does not disturb operations of equipment or processes within the system. (2) There is less disturbance of the are or plasma under analysis sinee the slit is in the end of a tapered nose cone of tungsten on the end of the analyzer. (3) A much closer approach to the boundary of or insertion into the are or plasma under analysis is possible without excessive drain on the power supply. (4) The are or plasma may be monitored continuously since the output of the analyzer is an oscilloscope trace adjusted to repeat times/sec. This provides rapid analysis particularly suited where conditions change rapidly. (5) Mass separation in regions of interest is sufiicicnt to provide a clear scan of individual ionic species such that even rnass defiect between similar e/m species can be determined.

This invention has been described by way of illustration rather than limitation, and it should be apparent that the invention is equally applicable in fields other than those described.

What is claimed is:

1. A velocity selector mass analyzer for continuously analyzing the ionic composition of a plasma source, comprising a source of plasma to be analyzed, a small, cylindrical outer tube, a tapered nose cone mounted in one end of said tube, said cone being provided with a narrow, ion entrance slit in the apex thereof, said slit being closely spaced from said plasma source, closure means mounted in the other end of said tube, a first pair of closely spaced, parallel plate electrodes insulated from and mounted within said tube, said first pair of electrodes being axially aligned with the axis of said tube and with said entrance slit of said nose cone and being closely spaced from said slit, a second pair of parallel plate electrodes insulated from and mounted within said tube and being parallel to and in alignment with said first pair of electrodes, a source of D.C. accelerating potential having a positive terminal and a negative terminal, means for connecting said positive terminal to ground and to said nose cone, means for connecting said negative terminal to one of said first pair of electrodes and to one of said second pair of electrodes, a source of AC. potential, means for connecting said A.C. source across said first pair of electrodes, means for providing a magnetic field, said field having a direction parallel to said electrodes, the spacing between said first pair of electrodes serving as a velocity selecting region for ions drawn therebetween through said nose cone entrance slit by said accelerating potential from said source of plasma under analysis, said spacing between said first pair of electrodes being provided with an electrostatic field by the potentials applied thereacross, the force on an ion due to said electrostatic field being made equal and opposite to the force on the same ion due to said magnetic field to prevent the ions in said velocity selecting region from striking either of said first pair of electrodes as said ions pass through said region, the other electrode of said second pair of electrodes serving as a collector plate for the ionic species separated in said velocity selected region by variations in said A.C. potential 'across said first pair of electrodes, a duel beam oscilloscope, means for connecting said collector electrode and said D.C. negative terminal and one terminal of said A.C.

potential to the vertical amplifier of said oscilloscope, and means for connecting the other terminal of said A.C. potential to said oscilloscope to provide a voltage index trace thereon, whereby an oscilloscope trace of the ionic composition of the source of plasma under analysis is con tinuously pnovided.

2. The analyzer of claim 1, and further including a second cylindrical inner tube coaxially mounted within and insulated from said outer tube, said first pair of electrodes and said second pair of electrodes being mounted within said inner tube, said first pair of electrodes extending beyond said inner tube toward said nose cone entrance slit.

3. A velocity selector mass analyzer for continuously analyzing the ionic composition of a plasma source comprising a source of plasma to be analyzed, an elongated tubular member provided with a tapered nose cone at one end and a plug at the other end, said nose cone being provided with a narrow, ion entrance slit in the apex thereof, said slit being disposed Within said plasma source, a first pair of closely spaced, parallel plate electrodes insulatingly positioned within said tube and axially aligned with and closely spaced from said ion entrance slit, said first pair of electrodes defining a velocity selecting region, a second pair of parallel plate electrodes insulatingly positioned Within said tube and being spaced from and parallel with said first pair of electrodes, one of said second pair of electrodes being a collector plate for the ionic species separated in said velocity selecting region, means for providing a magnetic field oriented to provide field lines parallel to the long dimension of said entrance slit, means for providing an electrostatic field in said velocity selecting region between said first pair of electrodes, the force on an ion due to said magnetic field and the force on the same ion due to said electrostatic field being made equal and opposite, a source of DC. potential, means for connecting said D.C. source between said nose cone and said first pair of electrodes for withdrawing and accelerating ions from said source of plasma to be analyzed through said velocity selecting region, a source of A.C. potential, means for connecting said A.C. source across said first pair of electrodes for separating the ionic species of the plasma source as they pass through the velocity selecting region according to their respective masses, and oscilloscope means connected to said collector electrode for continuously displaying the ionic composition of said plasma source.

4. The analyzer of claim 3, and further including mean-s disposed in said tubular member for cooling said electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 2,211,614 Bowie Aug.13, 1940 2,274,586 Branson Feb. 24, 1942 2,387,786 Waslrburn Oct. 30, 1945 2,927,232 Luce Mar. 1, 1960 2,945,119 Bl-ackman July 12, 1960 2,952,776 Schumacher et al Sept. 13, 1960 

1. A VELOCITY SELECTOR MASS ANALYZER FOR CONTINUOUSLY ANALYZING THE IONIC COMPOSITION OF A PLASMA SOURCE, COMPRISING A SOURCE OF PLASMA TO BE ANALYZED, A SMALL, CYLINDRICAL OUTER TUBE, A TAPERED NOSE CONE MOUNTED IN ONE END OF SAID TUBE, SAID CONE BEING PROVIDED WITH A NARROW, ION ENTRANCE SLIT IN THE APEX THEREOF, SAID SLIT BEING CLOSELY SPACED FROM SAID PLASMA SOURCE, CLOSURE MEANS MOUNTED IN THE OTHER END OF SAID TUBEM A FIRST PAIR OF CLOSELY SPACED, PARALLEL PLATE ELECTRODES INSULATED FROM AND MOUNTED WITHIN SAID TUBE, SAID FIRST PAIR OF ELECTRODES BEING AXIALLY ALIGNED WITH THE AXIS OF SAID TUBE AND WIHT SAID ENTRANCE SLIT OF SAID NOSE CONE AND BEING CLOSELY SPACED FROM SAID SLIT, A SECOND PAIR OF PARALLEL PLATE ELECTRODES INSULTATED FROM AND MOUNTED WITHIN SAID TUBE AND BEING PARALLEL TO AND IN ALIGNMENT WITH SAID FIRST PAIR OF ELECTRODES, A SOURCE OF D.C. ACCELERATING POTENTIAL HAVING A POSITIVE TERMINAL AND A NEGATIVE TERMINAL, MEANS FOR CONNECTING SAID POSITIVE TERMINAL TO GROUND AND TO SAID NOSE CONE, MEANS FOR CONNECTING SAID NEGATIVE TERMINAL TO ONE OF SAID FIRST PAIR OF ELECTRODES AND TO ONE OF SAID SECOND PAIR OF ELECTRODES, A SOURCE OF A.C. POTENTIAL, MEANS FOR CONNECTING SAID A.C. SOURCE ACROSS SAID FIRST PAIR OF ELECTRODES, MEANS FOR PROVIDING A MAGNETIC FIELD, SAID FIELD HAVING A DIRECTION PARALLEL TO SAID ELECTRODES, THE SPACING BETWEEN SAID FIRST PAIR OF ELECTRODES SERVING AS A VELOCITY SELECTING REGION FOR IONS DRAWN THEREBETWEEN THROUGH SAID NOSE CONE ENTRANCE SLIT BY SAID ACCELERATING POTENTIAL FROM SAID SOURCE OF PLASMA UNDER ANALYSIS, SAID SPACING BETWEEN SAID FIRST PAIR OF ELECTRODES BEING PROVIDED WITH AN ELECTROSTATIC FIELD BY THE POTENTIALS APPLIED THEREACROSS, THE FORCE ON AN ION DUE TO SAID ELECTROSTATIC FIELD BEING MADE EQUAL AND OPPOSITE TO THE FORCE ON THE SAME ION DUE TO SAID MAGNETIC FIELD TO PREVENT THE IONS IN SAID VELOCITY SELECTING REGION FROM STRIKING EITHER OF SAID FIRST PAIR OF ELECTRODES AS SAID IONS PASS THROUGH SAID REGION, THE OTHER ELECTRODE OF SAID SECOND PAIR OF ELECTRODES SERVING AS A COLLECTOR PLATE FOR THE IONIC SPECIES SEPARATED IN SAID VELOCITY SELECTED REGION BY VARIATIONS IN SAID A.C. POTENTIAL ACROSS SAID FIRST PAIR OF ELECTRODES, A DUEL BEAM OSCILLOSCOPE, MEANS FOR CONNECTING SAID COLLECTOR ELECTRODE AND SAID D.C. NEGATIVE TERMINAL AND ONE TERMINAL OF SAID A.C. 